1. Field of the Invention
The present invention generally relates to devices used to electromagnetically mark and locate obscured (buried) objects, and more particularly to a conduit end cap adapted to house a transponder or electronic marker enabling the later location of the non-conductive plastic conduit end using above-ground sensors.
2. Description of Related Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, optical fiber, power and television. It often becomes necessary to locate the ends of pipe or conduit stubs, in order to connect to new services, main extensions, access points such as cleanouts, branch conduit runs, etc. Conversely, is important to know with as much accuracy as possible, the approximate vicinity of the previously described application types in order to avoid disturbing them when digging or excavating for other purposes. Above ground marking devices may be installed after they are buried but they are often lost, stolen, or destroyed after a short period of use. Therefore, it is common to use underground marking devices or systems to enable the later location of a lost section or feature of an underground utility.
In the past, three different below ground approaches have been used to indicate the presence of ends of conduits, namely, warning tape, trace wires, trace wire, other conductors, and electronic marker systems. A warning tape is simply a band of plastic which is placed above the conduit before burial. These tapes are used to alert the excavation team of the presence of the conduit before any damage thereto might occur. As the backhoe or other mechanical digger excavates the site, it will hopefully uproot a portion of the warning tape prior to contact with the conduit. The primary disadvantage of (non-metallic) warning tapes is that they cannot easily be detected by any surface instrumentation.
A single trace wire is sometimes buried with a utility line. The trace wire is used as a conductor for an AC signal which is applied to the wire at one accessible end, and then acts as an antenna and radiates an electromagnetic field above ground along its entire length. The electromagnetic field may be detected with an appropriate receiver, and the underground path of the line thereby determined. The earliest cable locators used a single sensor which detects a single null or peak (depending upon the orientation of the sensor) as the unit passes near the cable. Many later devices use two or more sensors that combine the signals to provide an indication of conductor proximity. The most common sensors are ferrite-core antennas, i.e., inductors. Although the conduit itself may act as a conductor (i.e., when steel pipe or copper wire cabling is used), most conduits are non-conductive and therefore require a trace wire. There are three significant disadvantages in the use of a trace wire. First of all, it is necessary to provide above ground access to the trace wire in order to couple the AC signal thereto. Secondly, if a break occurs in the wire (due to excavation, or natural causes such as corrosion, earth movement or burrowing animals), then the wire becomes useless. Finally, the trace wire is too thin to imprint a warning message thereon, precluding any visual warning. Additionally, a receiver cannot easily distinguish the trace wire from any other conductor in the vicinity.
Electronic marker systems for locating buried objects are known in the art, and generally consist of two types, namely, active and passive markers (transponders). Active markers require the use of a power supply which amplifies a signal source (usually an AC signal). The signal is radiated by the underground marker and detected by a receiver unit above ground. Passive markers, in contrast, have no power supply, but rather operate in a resonant mode, responsive to a transmitted electromagnetic field.
A passive marker is basically a wire coil and capacitor surrounded in a protective envelope, which is then buried adjacent to the cable, pipe, or other object to be located. The marker is self-contained, with no external, accessible connections. Passive markers are activated by radiating a signal into the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held close to the surface (the transmitter portion of a transceiver). When the coil is directly over, or near, the passive marker (which is itself an inductive coil), the marker accepts energy within its bandpass and stores it, reaching a sustained amplitude during the transmission cycle. When the transmission cycle ends, the marker re-emits the energy at the marker's resonant frequency with an exponentially decaying amplitude. A second coil within the transceiver unit acts as a receiving antenna which detects the re-radiated energy, alerting the locating technician with an audible tone or other indicator means.
FIG. 1 illustrates several kinds of passive transponders for different applications. These include a small, near-surface marker 2 for locating a valve box, a medium size or mid-range marker 4 for locating a service drop (a loose coil of cabling deployed for future use), a full-range marker 6 for locating a more deeply buried conduit stub, and a so-called ball marker 8 for locating a conduit tee. The latter marker provides a spherical housing which supports the marker coil horizontally, regardless of the orientation of the housing (i.e., self-leveling), and is used for soil conditions which may result in significant shifting of the housing, such that the marker always provides a vertical location beacon (inductor axis).
Electronic markers, as well as warning tapes, are usually color-coded according to the particular type of utility line they mark. Specifically, gas line markers are yellow; telephone cable markers are orange; waste water tunnel markers are green; water pipe markers are blue; and power supply markers are red. Similarly, the passive marker is "coded" by tuning the coil for a specific resonant frequency. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power. In this manner, a locating technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal, which is not within the bandwidth for a telephone marker tuned for 101.4 kHz. Of course, these frequencies have been designated by convention, and are not meant to be restrictive.
There are hybrid systems wherein, for example, a signal is applied to a buried conductor (cable or trace wire), and coupled through the conductor to one or more markers buried adjacent the conductor. Also, a marker can be used to couple one conductor to another, so that the test signal may be conveyed to the second conductor without a direct physical connection.
While passive electronic markers have several advantages over warning tapes and tracing wires, they are still subject to certain limitations, primarily related to the desired resonant frequency of a particular marker. In a prior art marker, which is essentially an LC circuit, the resonant frequency f is given by the equation f=1/(2.pi..sqroot.LC) where L is the inductance of the wire coil and C is the capacitance of the capacitor. This frequency must be closely controlled in order to adhere to the foregoing tuning conventions and to provide a return signal of maximum amplitude; however, the actual frequency is affected by component construction, manufacturing tolerances, operating temperature, aging, placement and other factors.
One problem associated with using passive or active transponders for remote identification of buried utility structures is that, as alluded to regarding ball marker 8, the transponder must be properly oriented to maintain the coil axis in a generally vertical position. Current methods for locating service drops are highly dependent on field crews for correct placement of transponders for accurate locating. Even with correct placement procedures, movement can occur in a filled ditch or hole as a result of loose unpacked soil, freeze/thaw cycling, water erosion, and other causes. If the coil axis orientation deviates from the vertical, then the craftsperson searching for the marker will sense a peak signal at a position which does not correspond to the buried transponder, resulting in a mislocate, which can be costly if the excavation equipment damages any part of the underground utility due to the error. Although markers can be attached to the utility structure in the proper orientation using clamps, straps, tape, adhesive, etc., subsurface movement can still result in angular displacement of the markers. Also, initial placement of the markers requires careful use of tools such as levels, transits or plumb bobs.
While ball marker 8 overcomes this problem, it still has certain disadvantages. For example, the spherical design of the ball marker prevents its use in narrow spaces, such plow-in ditches. Furthermore, while angular displacement of the coil axis is avoided, there is still the possibility of translational movement of the housing itself, as the ball marker is not attached to the buried conduit. Such movement is especially troublesome when marking a specific point, such as a conduit end, as opposed to simply marking a portion of the length of the conduit. It would, therefore, be desirable to devise an improved method of accurately locating a conduit end without dependency on craft skill. It would be further advantageous if the method allowed the marker to be affixed to the buried conduit.